The present invention relates generally to dishwashing machines, and more particularly, to a control system specifically adapted for carrying out a method of controlling a conveyor-type commercial dishwashing machine.
Dishwashers fall into two generally distinct but somewhat overlapping categories, namely, domestic or home machines, and commercial machines such as are used in restaurants, institutions or other public facilities. This latter category is itself divisible into various types of commercial machines.
Two of the most common types of commercial machines are the single rack-type and the conveyor-type. The former includes a single chamber into which a rack of soiled ware can be placed. Within the chamber, the entire cleaning process including washing, rinsing and drying is performed on the rack. Multiple racks must be washed sequentially, with each rack being completely cleaned before the next can be operated upon. A conveyor-type machine, on the other hand, includes a conveyor for carrying individual items or entire racks of ware through multiple stations within the machine. A different operation is carried out at each station, such as washing, rinsing, or drying. Thus, multiple items or racks of ware can be placed on the conveyor and moved continuously through the machine so that, for example, while one item or rack is being rinsed, a preceding item or rack can be dried.
Examples of both of these types of commercial dishwashing machines can be found in U.S. Pat. No. 4,439,242 issued Mar. 27, 1984 to Hadden.
Dishwashers generally, and conveyor-type dishwashers in particular, are relatively energy intensive in their operation. For example, substantial quantities of water heated to at least temperatures of 120.degree. are introduced into the machine at the final rinse stage. Moreover, significant energy is required to drive the two or more pump motors which are typically rated in the range of one horsepower. A relatively large motor is also required to drive the conveyor, and electrical and steam heaters are required to maintain washing liquid at an adequate temperature within the machine tanks. A heater and blower motor are required for the drying operation, and various other components requiring energy input are present throughout the machine. Consequently, it is of great economic advantage to a machine owner for energy usage within the machine to be kept as low as possible.
One of the problems in making a conveyor-type dishwasher energy efficient results from the varying patterns of use that are typical with such machines. During peak periods, such as at meal times, a large quantity of dishware will typically pass through the machine. Therefore, it is not unusual at such times for ware items or racks to pass through the machine at closely-spaced intervals, so that the machine is essentially completely full for extended periods of time. In such a case, there is little waste of energy in having each component at each station of the machine in simultaneous operation.
At other times, such as between traditional meal periods, the dishwashing machine may be used only sporadically, although some ware may still need to be washed. In such a case, small numbers of items or single racks may be placed on the conveyor and moved through the dishwashing machine. In such a case, it makes little sense, for example, to operate the blower motor in the drying station while all the ware within the machine is still at the prewash station.
Some attempt has been made in the past to solve this problem on a limited basis. For example, in U.S. Pat. No. 4,285,352 issued Aug. 25, 1981 to McMahon et al, a conveyor-type dishwashing machine is disclosed in which a contactor switch is positioned along the conveyor path near the entrance to the final rinse station. A rack of ware moving along the conveyor physically contacts and closes the switch, whereupon operation of the final rinse station is commenced. Since heating of final rinse water is a substantial factor in energy demand, this approach conserves energy by reducing the quantities of heated water used within this station. Of course, such a system is limited to only the final rinse station, and is not effective for reducing energy consumption in other stations within the machine.
One possible, more comprehensive approach to reducing energy consumption would be to provide contact switches such as used in McMahon et al at the entrance and exit of each station within the machine. However, while such an approach might perhaps be workable for a machine whicn handles only ware in racks, it cannot be used with a conveyor-type machines having a flight-type conveyor in which individual items of ware can be carried directly by the conveyor without being placed within racks. In such a case, special contact switches or rows of switches would need to be positioned at each station so that a ware item positioned at any point across the width of the conveyor would initiate operation within a particular station. Moreover, physical contact of the switches with the ware items may result in chipping or other damage to breakable ware items such as china. While non-contact switches might seem to avoid this problem, such devices are not practical within the wet and humid environment within the machine, where quantities of water and steam are being moved about.
Another possible solution could be to have operation within each individual station commenced manually by the operator who monitors passage of ware items through the machine. However, careful consideration of such an approach will reveal its impracticality for ware items loaded into the machine in patterns any more complex than single, totally isolated items. Moreover, due to the possibility that entire operations could be inadvertently missed by certain ware items, thereby reducing the cleanliness of such items, it is doubtful whether such an approach could ever obtain approval from health and sanitation authorities.
It can be seen, therefore, that a need exists for a practical control system for use with a conveyor-type dishwasher in which operations within individual stations of a machine can be commenced only as needed. It is important that such a control system be operable with any loading pattern of ware onto the conveyor, regardless of whether the ware is within racks or placed on the conveyor as individual items. Such a system should be automated so that paricular operations are not inadvertently withheld from ware articles. At the same time, the system must be reliable, and malfunctions should not totally shut down the dishwashing capability of the institution at which the machine is used. Such an event could have disastrous consequences were it to take place during peak serving periods. Finally, the system must not be so complex that it substantially increases the cost of the dishwashing machine.